U.S. patent application number 14/609966 was filed with the patent office on 2015-05-21 for low cost synthesis of single material bifunctional nonprecious catalyst for electrochemical devices.
The applicant listed for this patent is Zhongwei CHEN, Fathy Mohamed HASSAN, Aiping YU. Invention is credited to Zhongwei CHEN, Fathy Mohamed HASSAN, Aiping Yu.
Application Number | 20150141666 14/609966 |
Document ID | / |
Family ID | 50027031 |
Filed Date | 2015-05-21 |
United States Patent
Application |
20150141666 |
Kind Code |
A1 |
CHEN; Zhongwei ; et
al. |
May 21, 2015 |
Low Cost Synthesis of Single Material Bifunctional Nonprecious
Catalyst for Electrochemical Devices
Abstract
A bifunctional catalyst for catalyzing both an oxygen reduction
reaction and an oxygen evolution reaction is provided, wherein the
catalyst comprises a doped graphene backbone having thiol
functional groups. A method for producing a bifunctional catalyst
is also provided.
Inventors: |
CHEN; Zhongwei; (Waterloo,
CA) ; Yu; Aiping; (Waterloo, CA) ; HASSAN;
Fathy Mohamed; (Kitchener, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CHEN; Zhongwei
YU; Aiping
HASSAN; Fathy Mohamed |
|
|
US
US
US |
|
|
Family ID: |
50027031 |
Appl. No.: |
14/609966 |
Filed: |
January 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/CA2013/050593 |
Jul 30, 2013 |
|
|
|
14609966 |
|
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61741869 |
Jul 30, 2012 |
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Current U.S.
Class: |
549/12 |
Current CPC
Class: |
H01M 4/96 20130101; C01B
32/23 20170801; H01M 12/06 20130101; C01B 32/194 20170801; H01M
4/8615 20130101 |
Class at
Publication: |
549/12 |
International
Class: |
H01M 4/96 20060101
H01M004/96 |
Claims
1. A bifunctional catalyst for catalyzing both an oxygen reduction
reaction and an oxygen evolution reaction, the catalyst comprising
a doped graphene backbone having thiol functional groups.
2. The bifunctional catalyst according to claim 1, wherein the
graphene is doped with heteroatoms.
3. The bifunctional catalyst according to claim 1, wherein the
graphene is doped with sulfur, nitrogen, oxygen, phosphorous or
boron.
4. The bifunctional catalyst according to claim 1, wherein the
graphene is doped with sulfur.
5. A method for producing a bifunctional catalyst for catalyzing
both an oxygen reduction reaction and an oxygen evolution reaction,
the method comprising: (a) mixing graphitic oxide with a source of
sulfur; and (b) heating the mixture to form graphene, wherein the
graphene is doped with sulfur and wherein graphene is provided with
thiol functional groups.
6. The method according to claim 5, wherein the source of sulfur is
sodium thiosulfate.
7. The method according to claim 5, wherein the source of sulfur is
thiourea.
8. The method according to claim 7, wherein graphene is further
doped with nitrogen.
9. A bifunctional catalyst obtained by a method according to claim
5, wherein the bifunctional catalyst comprises a sulfur doped
graphene backbone and thiol functional groups bonded thereto.
10. The bifunctional catalyst according to claim 9, wherein the
graphene is further doped with nitrogen, oxygen, phosphorous or
boron.
11. A cathode for use in an electrochemical device, the cathode
comprising a catalyst according to claim 1.
12. A cathode for use in an electrochemical device, the cathode
comprising a catalyst according to claim 4.
13. A cathode for use in an electrochemical device, the cathode
comprising a catalyst according to claim 9.
14. A cathode for use in an electrochemical device, the cathode
comprising a catalyst according to claim 10.
15. The cathode of claim 11, wherein the electrochemical device is
a metal-air fuel cell or a metal-air battery.
16. The cathode of claim 12, wherein the electrochemical device is
a metal-air fuel cell or a metal-air battery.
17. The cathode of claim 13, wherein the electrochemical device is
a metal-air fuel cell or a metal-air battery.
18. The cathode of claim 14, wherein the electrochemical device is
a metal-air fuel cell or a metal-air battery.
Description
CROSS REFERENCE TO PRIOR APPLICATIONS
[0001] This application is a Continuation of PCT Patent Application
Number PCT/CA2013/050593, filed Jul. 30, 2013, which claims
priority under the Paris Convention to U.S. Application No.
61/741,869, filed Jul. 30, 2012. The entire contents of the
aforementioned applications are incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a catalyst for use in
electrochemical devices. In particular, the invention relates to a
bifunctional catalyst and a method for producing the same.
BACKGROUND OF THE INVENTION
[0003] Electrochemical devices, such as metal-air batteries or
metal-air fuel cells, are very promising energy conversion
technologies that provide alternatives to the use of fossil fuels.
As is known in the art, typical metal-air batteries and fuel cells
comprise anodes that are formed using metals such as zinc (Zn),
aluminum (Al) and lithium (Li). During the discharge of such
batteries and fuel cells, oxidation of the metal occurs at the
anode, which releases electrons which are transported via an
external circuit to a cathode. At the cathode, an oxygen reduction
reaction occurs, converting oxygen from air and water from an
electrolyte into hydroxide ions. In zinc-air batteries in
particular, hydroxide ions then migrate through the electrolyte to
reach the anode where they form a metal salt (e.g. zincate), which
decays into a metal oxide (e.g. zinc oxide). As such, the metallic
anode gradually becomes depleted over time in a primary metal-air
battery or fuel cell, thus requiring a continuous supply of metal
for long term operation. However, the depletion of the anode can be
mitigated by introducing oxygen evolution reaction at the cathode
while the battery or the fuel cell is not being discharged, as this
result in the oxygen reduction reaction to occur at the anode,
which in turn causes metal to be regenerated at the anode. However,
the oxygen reduction reaction (ORR) and the oxygen evolution
reaction (OER) have large overpotentials and sluggish reaction
kinetics. Therefore, to realize large scale application of metal
air battery/fuel cells, improved catalysts are required.
[0004] Various approaches have been proposed to address the
abovementioned deficiencies, such as through the use of
bifunctional catalysts. Bifunctional catalysts are generally
catalysts that catalyze both oxygen reduction and oxygen evolution
reactions. For example, Jorissen (Ludwig Jorissen (2006);
"Bifunctional oxygen/air electrodes"; Journal of Power Sources 155
(1): 23-32) reviewed many bifunctional catalysts, which catalyze
both ORR and OER, made with various materials such as perovskite,
spinel and pyrochlore type mixed metal oxides. However, Jorissen
indicates that additional research is still needed in the field of
bifunctional catalysts. In another example, Lu et al. (Yi-Chun Lu
et al. (2010); "Platinum-Gold Nanoparticles: A Highly Active
Bifunctional Electrocatalyst for Rechargeable Lithium-Air
Batteries"; Journal of American Chemical Society, 132 (35):
12170-12171) describe a bifunctional catalyst based on platinum and
gold; however, the high cost of the catalyst discourages its large
scale implementation.
[0005] U.S. Publication No. 2007/0166602 to Burchardt describes
combining an oxygen reduction catalyst and various oxides (e.g.
CoWO.sub.4, La.sub.2O.sub.3) to obtain high bifunctional activity.
U.S. Publication No. 2007/0111095 to Padhi et al. describes using
manganese oxide contained in an octahedral molecular sieve as a
catalyst for metal-air cathodes.
[0006] Other approaches are also known which generally involve
selecting one catalyst for catalyzing the oxygen reduction reaction
and another for catalyzing the oxygen evolution reaction and
combining the two catalysts together to effectively obtain a
catalytic material that catalyzes both reactions. However, these
approaches add complications to electrode fabrication and increase
the cost of production. Furthermore, in most of these approaches,
the catalyst is either an expensive precious metal like platinum
(Pt) or gold (Au), or a mixture, composite or spinel of oxides
containing other expensive materials such as lanthanum oxide
(La.sub.2O.sub.3).
[0007] A further example of a catalyst is provided in Applicant's
co-pending PCT application number PCT/CA2012/050050, filed Jan. 27,
2012, the entire contents of which are incorporated herein by
reference.
[0008] Graphene is a material that is known to have unique
properties such as high chemical resistance and high electrical
conductivity, among others. More importantly, graphene is generally
prepared from graphite, which is inexpensive and abundant. It is
well known in the art to dope graphene with other elements to
increase its activity for use as a catalyst. Several researches
have explored the effects of doping graphene with elements such as
nitrogen and boron for various applications. Although the effects
of doping graphene with sulfur has been studied by Yang et al.,
(Zhi Yang et al. (2012); "Sulfur-Doped Graphene as an Efficient
Metal-free Cathode Catalyst for Oxygen Reduction"; ACS Nano 6 (1):
205-211) only its effects on oxygen reduction reactions (ORRs) were
studied. Furthermore, the sample preparation method used by Yang et
al. involved reacting aromatic compounds containing sulfur at high
temperatures, thus giving rise to a relatively high cost of
production as well as potential health and/or environment
issues.
[0009] The prior art also describes the beneficial effects of
mixing carbon materials with sulfur for electrochemical energy
applications. For example, U.S. Publication Number 2009/0311604 to
Stamm et al. describes mixing carbon material with sulfur to
prepare the electrodes for a lithium-sulfur battery. In another
example, in U.S. Publication Number 2012/0088154 to Liu et al.,
graphene-sulfur nanocomposites were used as an electrode material
for a rechargeable lithium sulfur battery.
[0010] Despite the various proposed approaches as discussed above,
there remains a need for a catalyst material that addresses at
least one of the deficiencies known in the art. For example, there
exists a need for cost effective catalysts that: (i) possess
improved activity, (ii) possess improved stability, and/or (iii) do
not contain expensive materials such as precious metals.
Furthermore, there exists a need for a method of producing
catalysts in a cost effective manner and which does not harm the
environment.
SUMMARY OF THE INVENTION
[0011] In one aspect, the present invention provides a bifunctional
catalyst for catalyzing both an oxygen reduction reaction and an
oxygen evolution reaction, the catalyst comprising a doped graphene
backbone having thiol functional groups.
[0012] In another aspect, the present invention provides a method
for producing a bifunctional catalyst for catalyzing both an oxygen
reduction reaction and an oxygen evolution reaction, the method
comprising mixing graphitic oxide with a source of sulfur, and
heating the mixture to form graphene, wherein the graphene is doped
with sulfur and wherein graphene is provided with thiol functional
groups.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The features of the invention will become more apparent in
the following detailed description in which reference is made to
the appended drawings wherein:
[0014] FIG. 1 is the chemical structure of a thiol functionalized
sulfur doped graphene (TFSG) according to one embodiment;
[0015] FIG. 2 is the chemical structure of a graphitic oxide (GO)
according to one embodiment;
[0016] FIG. 3 is a flowchart showing the conversion of graphite
into graphitic oxide (GO) according to one embodiment;
[0017] FIG. 4 is a flowchart showing the conversion of graphitic
oxide into thiol functionalized sulfur doped graphene (TFSG)
according to one embodiment;
[0018] FIG. 5 shows: a) a basic schematic diagram of a zinc-air
fuel cell; b) a SEM image of the TFSG; c) an image of an elemental
mapping for oxygen of a selected portion of the sample as shown in
b); d) an image of an elemental mapping for sulfur of the selected
portion of the sample; and e) an image of an elemental mapping for
carbon of the selected portion of the sample;
[0019] FIG. 6 shows: a) a spectrum obtained for TFSG using an x-ray
photoelectron spectrometer (XPS); b) a spectrum obtained for TFSG
using an x-ray photoelectron spectrometer (XPS); c) a spectrum
obtained for TFSG using raman spectroscopy; and d) a spectrum
obtained for TFSG using FTIR;
[0020] FIG. 7 shows: a) a plot showing ORR responses obtained from
sulfur doped graphene (SG), TFSG and Pt/C in a linear sweep
voltammetry test; b) a plot showing OER responses obtained from SG,
TFSG and Pt/C in a cyclic voltammetry (CV) test; c) a plot showing
the ORR responses obtained from TFSG at different rotation speeds;
and d) a plot showing the ORR responses obtained from SG at
different rotation speeds;
[0021] FIG. 8 shows: a) a plot showing the responses obtained from
a Pt/C catalyst on the first cycle ("before") and the 200.sup.th
cycle ("after") of a full range degradation test (FDT); b) a plot
showing the responses obtained from SG on the first cycle
("before") and the 200.sup.th cycle ("after") of a FDT; c) a plot
showing the responses obtained from the TFSG on the first cycle
("before") and the 200.sup.th cycle ("after") of a FDT; and d) a
graph showing the maximum current densities obtained in each FDT
test; and
[0022] FIG. 9 shows: a) a plot showing the discharge and charge
profiles obtained from a prototype zinc-air fuel cell incorporating
the TFSG catalyst at different cycles; b) a plot showing the charge
and discharge voltages obtained from a prototype zinc-air fuel cell
incorporating the TFSG catalyst over 200 cycles; c) a plot showing
the discharge and charge profiles obtained from a prototype
zinc-air fuel cell incorporating the Pt/C catalyst at different
cycles; d) a plot showing the charge and discharge voltages
obtained from a prototype zinc-air fuel cell incorporating the Pt/C
catalyst over 70 cycles; e) a plot comparing the maximum current
obtained for each fuel cell at different voltages; and f) a
schematic diagram of a prototype fuel cell.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The terms "comprise", "comprises", "comprised" or
"comprising" may be used in the present description. As used herein
(including the specification and/or the claims), these terms are to
be interpreted as specifying the presence of the stated features,
integers, steps or components, but not as precluding the presence
of one or more other feature, integer, step, component or a group
thereof as would be apparent to persons having ordinary skill in
the relevant art.
[0024] Generally, according to one aspect, the present invention
provides a metal-free bifunctional catalyst inspired from
structures of biological molecules (e.g. proteins with thiol
groups). According to one embodiment, a bifunctional catalyst for
catalyzing both an oxygen reduction reaction, during the
discharging phase of a electrochemical device, such as a metal-air
fuel cell or battery, and an oxygen evolution reaction, during the
charging phase, is provided wherein the bifunctional catalyst
comprises a doped graphene backbone having thiol functional
groups.
[0025] The bifunctional catalyst according to one embodiment is
shown in FIG. 1. As shown in FIG. 1, thiol groups (--SH), are
chemically bonded to carbon atoms of the doped graphene backbone.
In one embodiment, graphene is doped with heteroatoms, such as
sulfur, nitrogen, oxygen, phosphorous or boron. In one embodiment
of the invention, graphene is doped with sulfur. In one embodiment,
sulfur dioxide functional groups (--SO.sub.2) are also attached to
the doped graphene backbone as shown in FIG. 1. Furthermore, in the
embodiment shown in FIG. 1, graphene is shown as being doped with
both sulfur and oxygen.
[0026] According to another aspect, the present invention provides
a method for producing a bifunctional catalyst which catalyzes both
an oxygen reduction reaction and an oxygen evolution reaction, the
method comprising mixing graphitic oxide with a source of sulfur
and heating the mixture to form graphene, wherein graphene is doped
with sulfur and wherein graphene is provided with thiol functional
groups.
[0027] In one embodiment, sodium thiosulfate
(Na.sub.2S.sub.2O.sub.3) is used as the source of sulfur. The
reaction in one embodiment is generally performed by first mixing
graphitic oxide (GO) with sodium thiosulfate. The mixture of GO and
sodium thiosulfate is then heated to form graphene, wherein
graphene is doped with sulfur and graphene is provided with thiol
functional groups. As an example, the chemical structure of GO is
shown in FIG. 2. In one embodiment, GO and sodium thiosulfate are
mixed in water. The bifunctional catalyst produced according to one
embodiment of the method is shown as an example in FIG. 1.
[0028] In another embodiment, thiourea (CH.sub.4N.sub.2S) is used
as the source of sulfur. The reaction in one embodiment is
generally performed by first mixing graphitic oxide (GO) with
thiourea. The mixture of GO and thiourea is then heated to form
graphene, wherein graphene is doped with sulfur and graphene is
provided with thiol functional groups. In one embodiment, graphene
is further doped with nitrogen from thiourea.
[0029] In one embodiment, graphitic oxide (GO) is first prepared by
oxidizing graphene using oxidation methods which are well known in
the art, such as, for example, modified Hummer's method. An example
of a graphitic oxide produced by this method is shown in FIG.
2.
[0030] In one embodiment of the invention, the bifunctional
catalyst produced according to a method is further doped with
nitrogen, oxygen, phosphorous or boron. As will be appreciated by
the persons skilled in the art, graphene may be doped with the
above heteroatoms by choosing an appropriate source of sulfur.
[0031] In one embodiment, the bifunctional catalyst according to
the present invention is used as a cathode for use in an
electrochemical device to catalyze both oxygen reduction and oxygen
evolution reactions. For example, the cathode may be used in
metal-air batteries and fuel cells.
[0032] The performance of the bifunctional catalyst according to
one embodiment of the present invention was tested using cyclic
voltammetry (CV) and by incorporating a cathode comprising the
bifunctional catalyst into a prototype metal-air fuel cell as will
be described below. It was also shown through testing that the
thiol functional group attached to the doped graphene backbone was
found to create active sites on the surface of the catalyst
material that catalyze both oxygen reduction and oxygen evolution
reactions. In one embodiment of the invention, the bifunctional
catalyst is be used as a cathode without further treatment or
processing steps (i.e. as-prepared).
[0033] The bifunctional catalyst according to one embodiment is a
metal-free, non-precious (i.e. does not contain any precious
metals) catalyst for catalyzing both ORR and OER. In one
embodiment, the catalyst is a thiol functionalized sulfur doped
graphene (TFSG). In comparison to other commercially-available ORR
catalysts such as carbon supported platinum nanoparticles (Pt/C),
the bifunctional TFSG catalyst according to one embodiment of the
invention was found to have a comparable ORR activity and higher
OER activity. The bifunctional TFSG catalyst was also found to be
relatively durable in comparison to Pt/C, since the bifunctional
TFSG retained a majority of its catalytic properties even after a
number charge and discharge cycles. These findings were supported
by series of tests (e.g. microscopy, spectroscopy and cyclic
voltammetry) conducted on a TFSG sample produced according to one
embodiment as will be described.
SUMMARY OF FEATURES
[0034] Thus, as would be understood from the present specification,
the present invention, in one aspect, provides a one-step
hydrothermal synthesis of a metal-free bifunctional catalyst. In
one aspect, the metal-free bifunctional catalyst is a single
material, meaning that both oxygen evolution and reduction
reactions are catalyzed by the same material, as opposed to other
approaches known in the art where different OER and ORR catalysts
are mixed or coupled to one another to produce a composite and/or
hybrid bifunctional catalyst.
[0035] In a further aspect, these functional groups were also found
to have changed the ORR reaction mechanism to the more efficient 4
electron pathway, wherein hydroxide ions are produced, as opposed
to the less efficient 2 electron pathway wherein hydrogen peroxide
is produced as a by-product.
EXAMPLES
[0036] Aspects of the invention will now be illustrated with
reference to the following examples; however, it will be understood
that the scope of the invention is not to be limited by the
following examples.
Example 1
[0037] In this example, graphitic oxide (GO) was initially prepared
by using a modified Hummer's method, in which graphite is oxidized
to produce graphitic oxide as shown in FIG. 3. GO was produced, in
this example, by exposing graphite to a mixture of sulfuric acid,
sodium nitrate and potassium permanganate. Then, as shown in a
diagram in FIG. 4, 100 mg of graphitic oxide was dispersed in
approximately 35 mL of de-ionized water (DI) and an aqueous
solution of sodium thiosulfate (Na.sub.2S.sub.2O.sub.3, 200 mg) was
added to form a reaction mixture. The reaction mixture was then
transferred into a Teflon lined autoclave and was subjected to a
hydrothermal treatment at a temperature of 190.degree. C. for 24
hours. The mixture was then filtered to obtain a thiol
functionalized sulfur doped graphene (TFSG). TFSG was then washed
several times with deionized water and ethanol before being dried
in a vacuum oven at a temperature of 100.degree. C. The structure
and morphology of TFSG was analyzed by conducting sample analysis
using transmission electron microscopy (TEM), x-ray photoelectron
spectroscopy (XPS), Raman spectroscopy and Fourier transform
infrared spectroscopy (FTIR). In particular, the presence of the
thiol functional groups in the sample was confirmed using the above
techniques. The results of these analyses are shown in FIGS. 5 and
6. In particular, an SEM image of the TFSG is shown in FIG. 5b,
while the elemental mapping of elements O, S and C from a selected
area of the image in FIG. 5b are shown in FIGS. 5c, 5d and 5e,
respectively. The results from XPS, Raman spectroscopy, and FTIR
are shown in FIGS. 6a and 6b, 6c, and 6d, respectively. For
comparison purposes, a sample of sulfur doped graphene (SG) was
prepared using a high temperature pyrolysis of graphene oxide and
diphenyl sulfide and tested.
Example 2
[0038] The electrocatalytic activity and stability of various
catalyst samples, namely the TFSG, sulfur doped graphene (SG), and
Pt/C were each measured using rotating disc electrode (RDE)
voltammetry. Each catalyst was coated onto a glassy carbon working
electrode, which was then immersed in a glass cell containing an
electrolyte (0.1 M KOH). A reference electrode (a double junction
saturated calomel electrode (SCE)) and counter electrode (a
platinum wire) were also both immersed in the electrolyte.
[0039] Tests for comparing the oxygen reduction reaction (ORR)
activity of the TFSG to those of SG and Pt/C were conducted using
linear sweep voltammetry at a scan rate of 10 mV/s between 0.1 V to
-1 V (vs. SCE) in O.sub.2 saturated 0.1 M KOH at 900 rpm. The test
results are shown in FIG. 7a. In particular, it can be seen from
the test results that the TFSG catalyst exhibits a higher ORR
activity than SG and a comparable ORR activity to the state-of-art
ORR catalyst (i.e. a commercially available Pt/C catalyst). More
specifically, the half-wave potential of TFSG was measured to be
only 43 mV lower than that of Pt/C. Additional ORR tests were also
performed at different rotation speeds, namely 100 rpm, 400 rpm,
900 rpm and 1600 rpm, to observe the effects of electrode rotation
on the kinetics of the reactions for the TFSG catalyst and SG.
These test results are shown in FIGS. 7c and 7d, respectively.
[0040] For comparing the oxygen evolution reaction (OER) activity
of the samples, 40 cycles of cyclic voltammetry (CV) measurements
were taken at a potential scan rate of 50 mV/s over a potential
window of 0 to 1 V using rotating disk electrode with a rotation
speed of 900 rpm. The test was conducted in a nitrogen saturated
0.1 M KOH solution. In this example, measurements taken over 40
cycles were found to be sufficient for comparing the OER activity
of a commercial Pt/C catalyst to that of the TFSG catalyst. The CV
test results taken at the 5.sup.th cycle for each sample are shown
in FIG. 7b. In particular, it was observed during testing that the
Pt/C catalyst degraded rapidly, whereas the TFSG catalyst retained
a majority of its OER activity. As it can be seen from the above
test results, the TFSG catalyst displayed the highest OER activity
out of all the samples which were tested.
[0041] In this example, a kinetic analysis was conducted on a TFSG
sample to show that the presence of the thiol group changes the
reaction mechanism to a more efficient 4 electron pathway. As such,
it was found that the presence of the thiol groups in the TFSG
catalyst contributed to the high performance of TFSG in catalyzing
both ORR and OER as observed in the tests above.
[0042] In order to compare the durability of the TFSG catalyst
against other samples, each electrode coated with the sample was
subjected to a highly aggressive condition in a test called the
full range durability test (FDT). The tests were conducted by using
the CV test set up with the rotating disk electrode at a rotation
speed of 900 rpm. The potential range was selected such that the
range covers both oxygen reduction and evolution reactions (i.e.
from -1 V to +1 V (vs SCE)). The tests were performed in a nitrogen
saturated 0.1 M KOH at a scan rate of 50 mV/s. The test results
showing the response from the first cycle (labelled "before") and
the 200.sup.th cycle (labelled "after") for each of Pt/C, SG and
TFSG are shown in FIGS. 8a, 8b and 8c, respectively. FIG. 8d shows
a bar graph that compares the current densities (j) obtained at 1V
"before" and "after" for each sample. As can be seen from the above
FDT results, the TFSG catalyst's maximum current density decreased
to approximately 86% of its "before" value after 200 cycles, while
that of the Pt/C catalyst decreased to approximately 10% of its
"before" value. Accordingly, it was shown through the tests that
the TFSG catalyst is highly durable as a bifunctional catalyst in
comparison to the Pt/C catalyst.
Example 3
[0043] The performance of the TFSG as an air electrode (i.e.
cathode) was evaluated by incorporating it into a prototype
zinc-air battery. A polished zinc plate and a TFSG coated gas
diffusion layer (Ion Power Inc., SGL Carbon 10 BB, 2.5 cm by 2.5
cm) were used as an anode and a cathode, respectively. The TFSG
catalyst loading on the gas diffusion layer was approximately 0.5
mg catalyst/cm.sup.2 and the electrolyte used in the zinc-air
battery was 6 M KOH. Battery discharge and charge tests at various
currents were performed, along with repeated charge-discharge cycle
tests using a constant current. In particular, discharge and charge
profiles of the TFSG battery at different cycles (namely 1.sup.st,
100.sup.th and 200.sup.th cycles) are shown in FIG. 9a, and changes
in the battery charge and discharge voltages over 200 charge and
discharge cycles at 18 mA cm.sup.-2 are shown in FIG. 9b. For
comparison purposes, a similar device to the one described above
was constructed by incorporating a commercially available Pt/C
catalyst in the cathode instead. The device was tested using
identical test parameters to produce the discharge and charge
profiles shown in FIG. 9c and a plot of the battery charge and
discharge voltages over 70 cycles as shown in FIG. 9d. FIG. 9e
shows a bar graph of the current for the initial cycle and the
100.sup.th cycle for each device taken at 1V and at 2V. As shown
through these test results, the device incorporating the TFSG
catalyst maintained a relatively low overpotential in comparison to
the Pt/C device, thus displaying an increased performance. The
difference in performance between the two catalysts is particularly
evident when the current at various cycles is compared as in FIG.
9e.
[0044] FIG. 9f shows a schematic diagram of a prototype zinc-air
fuel cell. In one embodiment, the zinc-air fuel cell includes a
Perspex (PMMA) sheet 1 with holes extending therethrough for air
access, a pair of stainless steel current collectors 2 and 8, a
catalyst loaded gas diffusion layer 3 that acts as the cathode, two
separators 4 and 6 which forms an electrolyte chamber for
containing the electrolyte (6M KOH), a membrane (e.g. a Celgard
5550 membrane), a polished zinc (Zn) plate 8 that acts as the anode
and a Perspex (PMMA) sheet 9. In an aspect, the cathode is prepared
by first formulating a TFSG catalyst ink by ultrasonically
dispersing the TFSG catalyst and Nafion.RTM. in isopropanol, and
then spraying the TFSG catalyst ink on the side of the gas
diffusion layer that faces the electrolyte.
[0045] By way of example, a schematic diagram illustrating the OER
and ORR in a zinc-air fuel cell according to one embodiment is
shown in FIG. 5a.
Example 4
[0046] In this example, graphitic oxide (GO) was initially prepared
by using a modified Hummer's method as described above. Then,
graphitic oxide was dispersed in de-ionized water (DI) and thiourea
(CH.sub.4N.sub.2S) was added to form a reaction mixture. The
reaction mixture was then transferred into a Teflon lined autoclave
and was subjected to a hydrothermal treatment at a temperature of
120.degree. C. for 6 hours then 190.degree. C. for 18 hours. The
mixture was then filtered to obtain a thiol functionalized sulfur
and nitrogen doped graphene (TFSNG). TFSNG was then washed several
times with deionized water and ethanol before being dried in a
vacuum oven at a temperature of 100.degree. C. The structure and
morphology of TFSNG was analyzed by conducting sample analysis
using transmission electron microscopy (TEM), x-ray photoelectron
spectroscopy (XPS), Raman spectroscopy and Fourier transform
infrared spectroscopy (FTIR). In particular, the presences of the
thiol functional groups as well as the sulfur and nitrogen dopants
in the sample were confirmed using the above techniques.
Furthermore, the sample was used as an electrode in a prototype
fuel cell as described above to test its performance.
[0047] Although the invention has been described with reference to
certain specific embodiments, various modifications thereof will be
apparent to those skilled in the art. Any examples provided herein
are included solely for the purpose of illustrating the invention
and are not intended to limit the invention in any way. Any
drawings provided herein are solely for the purpose of illustrating
various aspects of the invention and are not intended to be drawn
to scale or to limit the invention in any way. The scope of the
claims appended hereto should not be limited by the preferred
embodiments set forth in the above description, but should be given
the broadest interpretation consistent with the present
specification as a whole. The disclosures of all prior art recited
herein are incorporated herein by reference in their entirety.
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